(1) Field of the Invention
The present invention relates to a method for manufacturing multilayer paperboard.
The invention also relates to an assembly suited for implementing the method and a paperboard product manufactured according to the invention.
(2) Description of Related Art
Paperboard is used for printing and its greatest use is in different types of packages. Important material qualities required from paperboard grades in packages are high strength and stiffness, as well as a sufficiently high quality of the printing substrate if text or pictures are to be printed on the package. Frequently, paperboards are also provided with barrier layers if they are used, e.g., for packaging liquids or products containing volatile components, such as coffee and other foodstuff. The surface quality of the printing substrate is determined by the requirements set on the quality of print on the product package, whereby luxury products obviously must be packaged in materials different from those used for shipping and consumer packages of bulk goods.
To make a paperboard sufficiently stiff, the paperboard must be produced rather thick, whereby a lot of raw material fiber stock is needed for manufacturing the paperboard. On the other hand, the higher the requirements set on the performance of the printing substrate, the more expensive raw materials must be used to render the product a sufficiently good brightness among other qualities. As the stiffness of paperboard is chiefly determined by its thickness, the specific material consumption and the raw material costs increase almost as a linear function of the product thickness and basis weight. Accordingly, it would be advantageous to have the surface of the paperboard made from a strong material of high density and good printing properties while the middle layer should have a low density. Conventionally, the density of the paperboard web is essentially constant over its entire cross section, because normal manufacturing techniques are incapable of producing a web with a substantially varying density across its thickness dimension. While folding paperboards do have a different density in the surface layer as compared to the density of the middle layer, even in these grades the variation in density is so small that there is no as effective way of reducing the material consumption of a thick paperboard as that offered by corrugated boards having a fluted middle layer. Hence, corrugated board is still preferred in the manufacture of packages due to the increasing demands on stiffer packaging materials.
Corrugated board is a layered product comprising two outer layers called liners and a corrugated middle layer known as fluting placed therebetween. In multilayer corrugated boards, the number of fluted middle layers may be greater than one, and the middle layers are separated from each other by planar paperboard layers similar to those forming the liner layers. While the shape, inter-crest spacing and height of the flutes may vary, fluting is invariably made in a perpendicular direction to the machine direction of the raw material webs. Since the entire web used in the middle layer is fluted without deforming the smooth surface continuity of the web sides, the fluted crests form linear and continuous bonding patterns on the surface whereto it will be glued. As a result, the rigidity of the middle layer becomes different in the lateral and longitudinal directions. In paperboard webs, the fibers orient themselves during web manufacture chiefly in the machine direction, that is, in the longitudinal direction of the web. As a result, the strength properties of the web become different in the cross-machine and machine directions. In a corrugated board, this difference is equalized by the greater stiffness of the middle layer in the direction of the flutes. Herein, the term “direction of flutes” is used in the meaning of the longitudinal direction of the crests and valleys of the flutes.
Although corrugated board is an advantageous packaging material, it has several shortcomings. The compressive strength of corrugated board varies in a wide range depending on whether the compressive force is imposed on a crest or a valley, and, furthermore, liner layers are not necessarily planar at all points, but they may have deformations caused by such factors as minor shrinkage after gluing, for instance. Of course, the surface layer properties of corrugated board are dependent on the thickness and quality of the liner, but nevertheless corrugated board is not generally considered to be a material of choice for printing with contacting printing methods, which curtails its use in premium-quality packages. While corrugated board grades as a rule also tend to be relatively thick, recently thinner corrugated boards with lower basis weights have been introduced commercially. The excessive thickness of this material limits its use in both printed products and small retail packages that are limited by their external dimensions. Accordingly, corrugated board does not usually serve as an alternative to paperboard at least in packages of premium quality.
One further shortcoming of corrugated board is its relatively complicated production process. A three-layer corrugated board is made by first fluting the web of the middle layer and then applying glue to the flute crests, whereupon the fluted medium is bonded by pressing to a first flat liner. The second flat liner is glued in a separate step to the thus formed two-layer base board. The glue is applied to the flute crests alone allowing the glue application to take place only by means of a hard-surfaced applicator such as a roll. This application method cannot assure a uniform bonding. The two-step production technique is expensive and a corrugated board production line becomes long and clumsy, thus requiring a large footprint and involving a high investment cost. In the manufacture of a multilayer corrugated board, the number of production steps increases in proportion to the number of layers in the product.